Bisphenols as condensation products of phenols and carbonyl compounds are starting substances or intermediates for the production of a large number of commercial products. Of particular industrial significance is the condensation product of the reaction between phenol and acetone, 2,2-bis(4-hydroxyphenyl)propane (bisphenol-A, BPA). BPA is used as a starting substance for the production of various types of polymer materials, such as e.g. polyarylates, polyether imides, polysulfones and modified phenol-formaldehyde resins. Preferred areas of application lie in the production of epoxy resins and polycarbonates.
Industrially relevant production methods for BPA are known and are based on the acid-catalysed reaction of phenol with acetone, wherein a phenol-acetone ratio of more than 5:1 is preferably established in the reaction. This reaction conventionally takes place continuously, and generally at temperatures of 45 to 110° C., preferably at 50 to 80° C. Both homogeneous and heterogeneous Bronsted or Lewis acids, such as, for example, strong mineral acids, e.g. hydrochloric or sulfuric acid, may be used as acid catalysts. Gel-like or macroporous sulfonated crosslinked polystyrene resins (acid ion exchangers) are preferably used, which may be both monodisperse and heterodisperse. Divinylbenzene is normally used as the crosslinking agent, but other crosslinking agents, such as divinylbiphenyl, may also be employed. In addition to the catalyst, a co-catalyst may be used. These are usually thiols, which carry at least one SH function and have a positive effect on both the selectivity and the reactivity of the reaction. The co-catalyst may be both homogeneously dissolved in the reaction solution and, in the case of the acid ion exchangers, fixed on the catalyst itself. Homogeneous co-catalysts are e.g. mercaptopropionic acid, hydrogen sulfide, alkyl sulfides, such as e.g. ethyl sulfide, and similar compounds. Fixed co-catalysts are aminoalkyl thiols and pyridylalkyl thiols which are ionically bonded to the catalyst, it being possible for the SH function to be protected and only released during or after fixing to the catalyst, such as e.g. in the case of dimethylthiazolidine. The co-catalyst may also be covalently bonded to the catalyst as alkyl or aryl thiol.
When phenol is reacted with acetone in the presence of acid catalysts, a product mixture is formed which, in addition to unreacted phenol and possibly acetone, primarily contains BPA and water. In addition, small quantities of typical by-products of the condensation reaction occur, such as e.g. 2-(4-hydroxyphenyl)-2-(2-hydroxyphenyl)propane (o,p-BPA), substituted indanes, hydroxyphenylindanols, hydroxyphenylchromanes, spirobisindanes, substituted indenols, substituted xanthenes and more highly condensed compounds with three or more phenyl rings in the molecular backbone. In addition, other secondary components, such as anisole, mesityl oxide, mesitylene and diacetone alcohol, may form by self-condensation of the acetone and reaction with impurities in the raw materials. For economic and technical reasons, the reaction is usually performed in such a way that a hundred per cent conversion of the acetone is not achieved and 0.1–0.6 wt. % acetone is still contained in the reactor discharge.
The above-mentioned by-products, such as water, and unreacted feedstocks, such as phenol and acetone, impair the suitability of BPA for the production of polymers and have to be separated off by suitable processes. High demands are made of the purity of the raw material BPA, particularly for the production of polycarbonate.
One processing and purification method for BPA takes place by separating BPA out of the reaction mixture in the form of an approximately equimolar crystalline adduct with phenol by cooling the reaction mixture, allowing the BPA-phenol adduct to crystallise out in a suspension crystallisation. The BPA-phenol adduct crystals are then separated from the liquid phase by suitable apparatus for solid-liquid separation, such as rotary filters or centrifuges, and passed on for further purification.
Adduct crystals obtained in this way typically exhibit a purity of >99 wt. % BPA, based on the sum of BPA and the secondary components, with a phenol proportion of approx. 40 wt. %. By washing with suitable solutions, which typically contain one or more components from the group consisting of acetone, water, phenol, BPA and secondary components, the adduct crystals may be freed of impurities adhering to the surface.
The liquid stream forming during the solid-liquid separation (mother liquor) contains phenol, BPA, water formed during the reaction and unreacted acetone, and is enriched in the secondary components typically forming during BPA production. This mother liquor stream is generally fed back into the reaction unit. To maintain the catalytic activity of the acid ion exchangers, water that has formed is removed by distillation, and any acetone still present is also removed from the mother liquor at the same time. The dehydrated reaction stream thus obtained is supplemented with phenol, acetone and optionally co-catalyst and is fed back into the reaction unit. However, the phenol may also be added, wholly or partly, before the dehydration. Alternatively, water and acetone may also be removed by distillation before carrying out the suspension crystallization of the BPA-phenol adduct. In the distillation steps mentioned, a partial quantity of the phenol present in the reaction solution and, depending on its nature, optionally part or all of the co-catalyst may also be removed by distillation at the same time. In a circulating operation of this type, the problem occurs that by-products from BPA production become concentrated in the circulating stream and lead to the deactivation of the catalyst system and to poorer product qualities. To avoid excessive concentration of secondary components in the circulating stream, a partial quantity of the circulating stream—optionally after partial or complete recovery of phenol by distillation—is discharged from the process chain as so-called BPA resin.
Furthermore, part or all of the circulating stream may be passed through a rearrangement unit filled with acid ion exchanger after the solid-liquid separation and before or after the separation of water and residual acetone. This unit is generally operated at higher temperatures than the reaction unit. In this rearrangement unit, some of the secondary components from BPA production present in the circulating stream are isomerised to BPA under the prevailing conditions, so that the total yield of BPA may be increased.
For the further recovery of secondary components, the resin may also be subjected to thermal, acid- or base-catalysed cleavage. The phenol released in this case, and optionally also isopropenylphenol, may be separated off by distillation and returned to the reaction.
The BPA-phenol adduct crystals obtained following the suspension crystallisation of the reaction solution and solid-liquid separation described above are fed into further purification steps, wherein the separation of phenol and optionally the reduction of the concentration of secondary components are achieved.
Thus, the BPA-phenol adduct crystals may be recrystallized for further purification from phenol, organic solvents, water or mixtures of the above solvents, which may optionally also contain BPA and its isomers, by a suspension crystallization. By selecting suitable solvents, the phenol present in the adduct crystals may also be completely or partly removed at the same time. Any phenol remaining in the BPA after the recrystallization is then separated off completely by suitable distillation, desorption or extraction methods.
Alternatively, the phenol may also be removed from the BPA-phenol adduct crystals by melting-out processes.
After separating off the phenol, a bisphenol-A melt is obtained, which may be used for the production of polycarbonate by the transesterification process (melt polycarbonate) without previous solidification. However, the bisphenol-A melt may also be solidified by known processes, e.g. by the prilling process or by flaking, for sale or further use. The melt may also be dissolved in sodium hydroxide solution and used for the production of polycarbonate by the interfacial polycondensation process.
The bisphenol-A that has been freed of phenol may optionally be subjected to another purification step, such as e.g. melt crystallization, distillation and/or recrystallization from phenol, water or an organic solvent, such as e.g. toluene, or mixtures of these substances, before further processing.
Where a sulfonic-acid ion exchanger and/or a sulfurous co-catalyst is used, it is observed that sulfurous compounds are carried through to the bisphenol-A end product and may thus lead to quality problems relating to color. These compounds may be degradation products of the sulfonic acid ion exchanger, such as sulfuric acid, arylsulfonic acids, oligomeric polystyrene sulfonic acids, fine-particle catalyst and similar. Co-catalysts that are homogeneously present in the reaction mixture, such as e.g. mercaptopropionic acid, silylmethanethiols, hydrogen sulfide, alkyl sulfides, such as e.g. ethyl sulfide, and similar compounds may be present in their original form or in the form of their degradation products or derivative products. Thus, the SH group may be alkylated, but compounds such as dimethyl sulfide or disulfidic compounds, elemental sulfur, thio ethers, H2S as a cleavage product from the co-catalyst and other compounds may also be involved.
The same applies to co-catalysts fixed to the catalyst, such as e.g. thiazolidines, amino(alkyl) thiols, (the above compounds may also be used in the form of their hydrochlorides and/or with an SH function masked e.g. by acyl, benzyl or tert.-butyl groups, but in which this may readily be released again), pyridyl(alkyl) thiols, alkylcarbamoylalkyl thioesters, covalently bonded aryl or alkyl thiols. These may be split off and/or destroyed and/or simply washed out in the case of the ionically bonded co-catalysts.
In the further course of bisphenol-A production and handling, particularly when bisphenol-A and product streams containing bisphenol-A are subjected to thermal stress, this may lead to undesirable reactions, cleavages and the like, which may, for example, lead to a reduction in product purity, measured as the p,p-BPA content in the end product, as well as to the colour formation already mentioned.
The object of the present invention was therefore to provide a process for the production of bisphenol-A in which the sulfur content in the bisphenol-A end product is reduced. It was also the object of the present invention to provide bisphenol-A having a residual sulfur content of <2 ppm.